Lock runs command (default rc) with arguments while holding lockfile
open and (over)writing at least one byte each minute to keep the
exclusive–access lock alive. If lockfile doesn't already have the
exclusive–access bit set in its mode, the exclusive–access bits
are set in its mode and qid.type.

Under –w, lock waits for exclusive access to lockfile instead of
just trying once.

Lock sets /env/prompt to contain the name of the lock file.

EXAMPLES

Build a replica(1) database while preventing collisions with other
occurrences.